| Developing magnesium hydroxide products using Magnesium resources in salt lakes will contributes a lot to magnesium industry. It is also a good approach to solve the so-called "magnesium crisis" of some salt lakes in China, and to improve on the comprehensive utilization of lithium, boron and other associated valuable elements. At present, ammonia-brine method and ammonia-lime-brine combined way attracted attention on comprehensive utilization of brine resources, but there are many obstacles to acquire magnesium hydroxide products due to its fine particle size, the difficulty to solid-liquid separation and the purity problems of the product. In order to develop high-purity magnesium hydroxide products, and to optimize the use of resources of lithium and boron, it is necessary to clarify the effect of lithium and boron impurities on the magnesium hydroxide precipitated process. The transferring behavior of lithium and boron in the ammonia-brine magnesium recover process are also concerned.In this thesis, the effect of lithium and boron on crystallization properties of magnesium hydroxide and the crystal growth mechanism were studied using X-Ray diffraction (XRD), scanning electron microscopy (SEM), laser particle size analysis, Fourier transform infrared (FTIR), thermal gravimetric analysis (TG), differential scanning calorimetry analysis (DSC) and theoretical calculation methods. The precipitate properties in the ammonia (gas)-magnesium chloride (solution) reaction system with LiCl or H3BO3as impurities and the preparation of magnesium hydroxide crystalline with special morphologies using hydrothermal treatment and homogeneous precipitation methods were investigated. This work has academic and practicality value in some degree on the comprehensive utilize of Li, B and Mg resources and development of magnesium hydroxide functional materials.The main results of this thesis are as follows:(1) At room temperature, magnesium hydroxide was prepared by ammonia gas and magnesium chloride with LiCl impurity.The results of XRD, FTIR Spectrum and SEM were confirmed that the effect of Li was not obvious on the crystal phase and morphology of Mg(OH)2, but the surface charge property of Mg(OH)2colloidal particles was changed. The average specific surface area of the precipitation reduced to one twentieth of blank sample. With the increase of LiCl concentration from0.0375mol·L-1to3.0mol·L-1, the activation energy of decomposition reaction increased firstly then decreased, and the decomposition temperature drops from380℃to325℃. It could be concluded that lithium ion did not transfer from solution to Mg(OH)2precipitation.(2) A similar preparation system with H3BO3was performed in the same way mentioned in (1). Characterization results shows that the effect of B was significant, the precipitate phase changed from Mg(OH)2crystalline phase to amorphous phase consisted of complicated boron-magnesium compounds. The average particle size increased about seventy times compared with blank sample. Decomposition behaviors were significantly affected by boron impurity:the decomposition temperature increased firstly and then become stable; weight loss rate of TG curve reduced from25%to13.7%, which were lower than weight loss rate27%of blank sample; the decomposition activation energy and DSC peak temperature were higher than that of blank sample. The research results showed that boron component could transfer from liquid phase to solid phase by forming of boron-magnesium compounds.(3) The concurrent effect of LiCl (constant content) and H3BO3(variable content) on the precipitation process were studied. The complicated boron-magnesium compounds were found by XRD and FTIR. SEM demonstrated that the concurrent effect precipitation had morphology was same as boron impurity, which confirmed that boron had more significant affect than lithium. The particle size of the precipitation undulated with the increase of boron due to the complex structures of boron in solution. The change trend of decomposition activation energy was also similar, but the DSC peak temperature was lower than that of boron impurity system, which meant that lithium impurity could decrease the decomposition temperature of complicated boron-magnesium compounds. (4) Magnesium hydroxide with hexagonal plate and fabric veins morphology were prepared using hydrothermal method and homogeneous precipitation method respectively. The results of XRD and FTIR verified the crystalline phase of Mg(OH)2precipitation. The average particle size increased slowly with hydrothermal treatment time; the precipitation of large particles size with wide distribution was prepared by homogeneous precipitation method with ammonium sulfide. Relative sizes of specific crystal planes (001) and (101) showed that (001) was a preferential growth crystal plane than (101) in hydrothermal method, on the contrary,(101) was preferential plane in homogeneous precipitation method with the ammonium sulfide, and crystal growth of Mg(OH)2prepared with ammonia showed a competition situation between (101) and (100) crystal planes. The relative ratio of growth units accumulated on surface (101) and (001) were calculated and surface (101) of Mg(OH)2prepared by homogeneous precipitation method showed preferential growth tendency.(5) Ideal crystal morphology of Mg(OH)2was simulated with the BFDH method, the stability order of ideal crystal planes was (001)>(100)>(101), which was coincidence with the crystal prepared by hydrothermal treatment method. Quantum chemistry method was applied to calculate the total energy of Mg(OH)2crystal (001),(100) and (101) slab respectively, and the order of total energy was E(001)<E(100)<E(101), it was confirmed that Mg(OH)2crystal growth units should be easier to grow on (101) surface than (001). The calculation results also gave the driving force microscopic explanation of Mg(OH)2transform mechanism from colloid transform to hexagonal shape. |
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